1. Field Of The Invention
The invention relates to a downhole acoustic transducer for use in a well such as an oil well for the purpose of generating or detecting acoustic waves. Depending on circumstances, the transducer constitutes a seismic source or a receiver.
2. Description Of The Prior Art
Downhole seismic sources are used to determine underground geological characteristics in the region surrounding the well in which the source is placed. They are designed to create acoustic waves and to transmit these waves as fully as possible to the inside wall of the well. Receivers such as accelerometers, geophones, or hydrophones detect these acoustic waves after they have passed through the subsoil. After treatment, the measured waves serve to determine the characteristics of the underground formations passed through.
In a first measurement technique referred to as the "cross well seismic" technique, the receivers are placed in wells other than the well containing the source. In a second measurement technique referred to as the "single well seismic" technique, the receivers are placed in the same well as the source. .
Other configurations are also possible, and in particular the receivers may be placed on the surface of the ground while the transmitters are placed down the well.
U.S. Pat. No. 4,525,645 describes a piezoelectric seismic source constituted by piezoelectric rings that are polarized in a radial direction and glued to the inside of a metal tube which is closed at its ends. This source suffers from the drawback of not including a pressure balancing system for balancing the pressure between the inside of the tube and the external pressure. Consequently, it is necessary to increase the thickness of the metal tube to increase its mechanical strength for withstanding pressure. Given that any increase in this thickness also increases the stiffness of the tube, such an increase in wall thickness also reduces the change in volume of the source as generated by expansion of the piezoelectric rings when under voltage stimulation.
Another drawback of seismic sources of this type stems from the use of piezoelectric rings that are polarized radially. An improvement in acoustic performance can only be obtained by increasing the radial thickness of the piezoelectric rings which would require a proportional increase in the voltage applied to the rings. Given that the electrical voltage available at the bottom of a borehole is limited (e.g. to about 3000 volts), this leads in practice to a limitation on the acoustic performance of the source. Given these various limitations due to its design, it is practically impossible to make a seismic source of the type described in U.S. Pat. No. 4,525,645 capable of withstanding pressure greater than about 400 bars, while still having acceptable acoustic performance.
Because of the limited power of such a source, it is generally excited at its resonant frequency which is about 1 kHz under operating conditions. However, operating at its resonant frequency gives rise to seismic waves of relatively long duration. The signals delivered by the receivers therefore require complex processing in order to separate successive seismic events in time.
U.S. Pat. No. 4,651,044 proposes a second piezoelectric seismic source, derived from the preceding source. In the second source, the piezoelectric rings and the tube in which they are glued are open over their entire length parallel to their axis, and the assembly is placed inside an external sealing sheet. In a variant, the piezoelectric rings are replaced by a stack of piezoelectric pellets radially oriented relative to the tube so as to bear there against at locations situated at equal distances from the axially formed slot inside the tube.
However, in this case also, there is no balancing between the pressure inside the source and the external pressure. Consequently, when the external pressure increases, the tube closes and the source behaves like the first source. In addition, having a slot along the tube rapidly makes it very difficult to maintain sealing with increasing external pressure. For the same reasons as in the source described in U.S. Pat. No. 4,525,645, the second piezoelectric source has limited acoustic performance. It too therefore requires to be operated at resonance, which leads to complex processing of the signals delivered by the receivers.
An object of the invention is to provide a high-performance acoustic transducer suitable for use as a borehole seismic source.
A further object is to provide a transducer capable of providing a high power while operating outside its resonant frequency, whereby subsequent processing of the detected signals is simplified.
A further object is to provide a transducer capable of withstanding high pressure and high temperature encountered in boreholes.
A preferred embodiment of the invention is described below by way of non-limiting example and with reference to the accompanying drawings, in which: